1. Field of the Invention
The present invention relates to a DC drive type light emitting device and a method of producing the light emitting device, in particular, a light emitting device which can be driven at a low voltage and has high luminous efficiency and a method of producing the light emitting device.
2. Description of the Related Art
An electroluminescence (hereinafter, also referred to as “EL”) device is a conventionally known example of a thin light emitting device for use in, for example, the backlight of the display of a mobile phone.
EL devices are classified into dispersion type EL devices and thin-film type EL devices depending on a difference in the basic structure of EL devices to be caused to emit light. A dispersion type EL device using a zinc sulfide powder as a phosphor is most generally used.
The dispersion type EL device emits light on the basis of the following principle: the particles of the phosphor for the dispersion type EL device are dispersed in a dielectric substance, and an AC voltage and a DC voltage are applied between electrodes which are placed on both sides of the dispersed phosphor and at least one of which is transparent so that light is emitted.
The dispersion type EL device is characterized in that the device can be formed on a flexible film (such as PET or TAC) substrate by a simple method such as screen printing or any one of various coating methods.
Accordingly, the dispersion type EL is a light emitting device which is of low cost and is suitable for large area displays, and has been used in, for example, an advertising display. However, when considering that it is a surface light emitting device, the device is problematic because of its low brightness and short lifetime as compared to those of a thin-film type EL device.
At present, an AC operated double insulation structure device has been mainly put into practical use in the field of the thin-film type EL devices. A thin-film type EL device is characterized in that an EL light emitting layer using zinc sulfide as its base material is covered with two insulating layers. This structure allows a high electric field to be stably applied to the light emitting layer.
However, as is apparent from the structure of the double insulation structure device, the device is operated only by an alternating current.
On the other hand, a thin-film type EL device that emits light upon application of a direct current requires a driving power supply simpler than that of an AC operation type device with a load close to a purely capacitive load, so the thin-film type EL device is expected to be efficiently driven.
However, nearly all attempts that have been made heretofore are put in the category of a collision excitation type in terms of the excitation mechanism of a phosphor.
This type essentially requires a high electric field region of 105 to 106 V/cm in a light emitting layer, so dielectric breakdown makes it difficult to maintain a high electric field stably.
In addition, owing to this high DC electric field, an ionized impurity in the light emitting layer moves in the direction of the electric field, though the movement is slow. The movement changes the electric field distribution, and shifts the operating characteristics. The change and the shift are also responsible for the instability of the electric field.
In view of the above, a carrier injection/recombination type thin-film light emitting device capable of operating at a low DC voltage of about 10 V has been recently developed, and it has been reported that several devices were successfully developed.
For example, a carrier injection/recombination type device using an Al—ZnS—CuGaS2 diode structure (Japanese journal of applied physics, vol. 31, p. L1606 (1992)) and a carrier injection/recombination type device using a p-type CuGaS2/n-type ZnO:Al heterojunction diode structure (Journal of physics and chemistry of solid, vol. 66, p. 1868 (2005)) have been reported.
However, a device using any one of the structures described in those documents has extremely low luminous efficiency. For example, in the technique disclosed in Japanese journal of applied physics, vol. 31, p. L1606 (1992), light is barely emitted by applying a voltage of 19 V at a temperature of 90 K.
This is probably due to the poor lattice matching of the pn junction and the low carrier injection efficiency of the carriers injected into a light emitting layer.
A method involving providing the inside of a light emitting device with plural points to each of which an electric field concentrates has been conventionally known as a method of improving the carrier injection efficiency. A device to which the method is applicable is not limited to an inorganic EL device. For example, in Japanese Patent Application Laid-Open No. 2001-348296, a needle-like diamond film is used as a material for a hole transporting layer of an organic EL device.
In this case, when a voltage is applied between a hole injecting electrode and an electron injecting electrode, a needle-like structure is formed on the surface of the diamond film, so the hole injection efficiency of the holes injected from the needle-like structure becomes high. As a result, an organic light emitting device having extremely high emission intensity can be obtained.
Examples of a method of providing the inside of a light emitting device with a fine structure serving as an electric field concentration point include the following.
The examples include a method based on a plasma treatment described in Japanese Patent Application Laid-Open No. 2001-348296, a method of forming a fine structure involving the employment of, for example, lithography, and a method of forming a fine structured material involving the utilization of a structure formed in a self-organized manner.
However, in a material for use in a known DC drive type inorganic EL device, it is difficult to uniformly form such fine structure as described above in a film surface with good reproducibility. Accordingly, stable light emitting property cannot be obtained.
In particular, a method involving the employment of, for example, a plasma treatment or lithography cannot provide good interfacial property, with the result that the carrier injection efficiency of the carriers injected reduces.
In view of the above, an object of the present invention is to provide a DC drive type inorganic light emitting device excellent in luminous efficiency. Another object of the present invention is to provide a method of producing the light emitting device.
To solve the above-mentioned problem, according to the present invention, there is provided a light emitting device, including: a substrate; and a first layer and a second layer laminated on the substrate, in which: the second layer is formed of: a first portion containing Zn and at least one element chosen from S and Se as its constituent elements; and a second portion containing at least one element chosen from Cu and Ag and at least one element chosen from S and Se as its constituent elements; the first layer is a light emitting layer formed of at least one element chosen from S and Se and of Zn; and in the second layer, the second portion has a cross section parallel to the substrate which tapers toward the first layer.
Further, according to the present invention, there is provided a method of producing a light emitting device in which a first layer and a second layer are provided to be adjacent to each other on a substrate, the method including: forming the first layer containing Zn and at least one element chosen from S and Se; and forming the second layer containing at least one element chosen from S and Se, at least one element chosen from Cu and Ag, and Zn, in which at least one of a feeding amount of the at least one element chosen from Cu and Ag and a feeding amount of Zn is changed with time.
Further, according to the present invention, there is provided a method of producing a light emitting device in which a first layer and a second layer are provided to be adjacent to each other on a substrate, the method including: forming the first layer containing Zn and at least one element chosen from S and Se; and forming the second layer containing at least one element chosen from S and Se, at least one element chosen from Cu and Ag, and Zn, in which: at least one of a feeding amount of the at least one element chosen from Cu and Ag and a feeding amount of Zn is changed with time; and the second layer is distributed being separated into a first portion containing Zn and at least one element chosen from S and Se as its constituent elements, and a second portion containing the at least one element chosen from Cu and Ag and the at least one element chosen from S and Se as its constituent elements, and is formed so that the second portion has a cross section parallel to the substrate which tapers toward the first layer.
According to the present invention, the presence of the portion that tapers toward the light emitting layer enables the stable formation of a structure excellent in interfacial property in the inside of the light emitting device.
As a result, an interface having such a shape that an electric field locally concentrates can be stably formed with good controllability. Accordingly, charge can be effectively injected into the light emitting layer, and light can be emitted at a low voltage.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.